(1) Field of the Invention
The present invention relates to a method of producing a support for photographic paper. More particularly, the present invention relates to a method of producing a support for photographic paper provided with a coating layer of a waterproof resinous material in which a pigment is in the form of fine particles evenly dispersed therein, and exhibiting a superior resolving power.
(2) Description of the Related Arts
Formerly, baryta paper was used as a support for photographic paper, but recently a waterproof support composed of a substrate sheet consisting of paper and coating layers comprising a polyolefin resin and formed on two surfaces of the substrate sheet has become widely used for such photographic paper.
The waterproof support is advantageous in that, in a developing step for photographic paper prepared from the waterproof support, the time needed for washing and drying the photographic paper can be shortened, a ferrotype procedure can be omitted, the consumption of chemicals can be reduced, the rigidity and mechanical strength of the photographic paper can be maintained at a high level, and the photographic paper can be protected from staining due to a penetration of chemicals therein.
In the production of photographic paper having a superior resolving power by coating a photographic emulsion on a surface of the waterproof support, it is important to enhance a light-shielding effect of the waterproof resin coating layer of the support. Usually, for this purpose, a white pigment, for example, titanium dioxide powder, is dispersed in the waterproof resin coating layer on the support surface to be coated with the photographic emulsion.
To uniformly disperse the pigment such as titanium dioxide powder in the waterproof resin, a master batch is prepared by melt-kneading a mixture of 20% to 60% by weight of the pigment, for example, titanium dioxide powder, with the balance comprising the waterproof resin, by using a Banbury mixer, kneader or monoaxial knead-extruder, and mixing the master batch with an additional amount of the waterproof resin to adjust the content of the pigment in the resultant mixture to a predetermined level of 5% to 20% by weight. This method is disclosed in Japanese Examined Patent Publication (Kokoku) No. 61-26649.
The preparation of the master batch by using the Banbury mixer or kneader is advantageous in that the resin can be uniformly mixed with the pigment at a high mixing efficiency, but is disadvantageous in that the procedure is carried out discontinuously, i.e., batchwise, and thus has a low productivity.
The preparation of the master batch by using the monoaxial knead-extruder is advantageous in that the procedure is effected continuously, and thus at a high productivity, but is disadvantageous in that the fine pulverization of the pigment particles and uniform dispersion of the pigment in the resin are unsatisfactory, and thus the light-shielding effect of the resultant resin coating layer in the support for the photographic paper is insufficient and the resolving power of the resultant photographic paper is poor.
As stated above, the conventional processes for producing the support for photographic paper are usually unsatisfactory in both or either the productivity of the process and the quality of the resultant product, especially the fine pulverization and uniform dispersion of the pigment. Therefore, there is a strong demand for a new method of producing a support for photographic paper having a high quality, at a high productivity.